Gravity is strong enough to hold us to the surface of this planet, and powerful enough to make achieving orbit a tricky proposition. But why doesn't it crush us completely? It's because gravity is so much weaker than the other forces in the universe, something physicists have always struggled to explain.

But now, two physicists, Lisa Randall and Raman Sundrum, have come up with a model to explain gravity, that adds extra dimensions and traces back to the beginning of the universe.

We spend every day of our lives rooted to the Earth because of gravity, so it's natural that most of us consider it a powerful force in the universe. Not so.

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Think about it: The force of the entire Earth pulling down on a pin or a paperclip can be overcome with a small fridge magnet. Static electricity can make fabric and strands of hair stubbornly defy gravity — and all that takes is a short walk in a wool hat. Entire groups of people can be lifted off the ground for hours with a large balloon full of hot air. If gravity were as strong as the electromagnetic force, or the strong and the weak forces in an atomic nucleus, we'd all be a very finely distributed sludge over the surface of the earth.

So why aren't we sludge? Nobody is quite sure. But two physicists, Lisa Randall and Raman Sundrum, came up with a basic idea. First of all, picture all of reality as we know it — three dimensions of space and one of time — crammed into a single sheet of material, called a brane. It's floating in the rest of a vast universe that we don't perceive. We don't perceive this universe because the photons that let us see light, are crawling along our brane with us. In fact, most of the stuff that affects us is locked on the brane we happen to occupy.

But not gravity. Gravitons are small and so-far unseen particles that make up what we call the 'force' of gravity. They buzz like bees around a different brane, one that runs close to our own but doesn't touch. What does touch is the gravitons, though their effects are diluted. Over on the gravity brane, gravitons are as strong as any of the other forces that we've observed so far. Once they interact with our dimension, though, they are so weak that we can walk around on the earth without being squished, and the earth can roll around the sun without being sucked in, and the sun can spin around the black hole in the galactic center without . . . you get the picture.

The entire concept of branes may sound like untestable speculation, but it could contain answers about the early inflation of the universe, if we dig deeper into the idea. Why are branes separated from each other at all? Thirteen point seven billion years ago, when the entire universe was squeezed into a size no bigger than a pinhead, everything had to be bundled together. If branes exist, they could have started out tangled up with each other, and have been subsequently separated out because they were blown away from each other during the early expansion of the universe. This is one of the reasons huge facilities have been built to catch and measure gravitational waves.

If we manage to observe gravity waves, we'll undoubtedly know a little more about our observable universe. We could learn a little more about branes, the unobservable bits of the universe. And if we can tease out information about that, it could lead us all the way back to understanding how both the observable and unobservable universe came to be. A simple question about why gravity doesn't collapse us all into mush leads to a theory that might give us information about multi-dimensional space and the origin of the universe itself.